Mettre le doigt...

Mettre le doigt can be used literally to talk about putting a finger over the hole of a musical instrument, such as a flute or trumpet, but it's more commonly used in one of several idiomatic French expressions:

The French proverb:Entre l'arbre et l'écorce il ne faut pas mettre le doigt literally says "Between the tree and the bark you mustn't put your finger," but what it really means is that you shouldn't meddle in other people's business.